Method for removing  a polysilicon protection layer on a back face of an igbt having a field stop structure

ABSTRACT

Disclosed is a method for removing a polysilicon protection layer ( 12 ) on a back face of an IGBT having a field stop structure ( 10 ). The method comprises thermally oxidizing the polysilicon protection layer ( 12 ) on the back face of the IGBT until the oxidation is terminated on a gate oxide layer ( 11 ) located above the polysilicon protection layer ( 12 ) to form a silicon dioxide layer ( 13 ), and removing the formed silicon dioxide layer ( 13 ) and the gate oxide layer ( 11 ) by a dry etching process. The method for removing the protection layer is easier to control.

FIELD OF INVENTION

The present invention relates to semiconductor technology, and in particular to a process for removing a polysilicon protection layer on a back face of an IGBT structure having a field stop structure.

BACKGROUND

IGBT (Insulated Gate Bipolar Transistor) is a compound full-controlled voltage-driven power semiconductor device composed of a bipolar transistor and an insulated gate field effect transistor, and it has the advantages of both high input impedance characteristic of MOSFET and low forward voltage drop characteristic of GTR, and is characterized by low drive power and low saturation voltage drop. Therefore, IGBT is quite applicable to converter systems with a direct voltage of 600V or above, such as alternating current dynamos, variable-frequency drives, switch-mode power supplies, lighting circuits, traction drives and other areas.

In a manufacturing process of an IGBT, a silicon dioxide (SiO₂) layer and a polysilicon protection layer on the back face of the IGBT having an FS (Field Stop) structure are removed by means of wet etching of silicon on the back face (SEZ) in the backend process of the entire manufacturing process. As this conventional removing method is implemented in the backend process where a front face metal layer has already been formed, there is a risk of metal contamination. In addition, since SEZ can etch silicon, it may be easy to corrode the formed field stop layer due to inadequate control when removing the SiO₂ layer.

SUMMARY OF THE INVENTION

In view of the foregoing, the present invention provides a method for removing a polysilicon protection layer on a back face of an IGBT having a field sop structure. The method comprises thermally oxidizing the polysilicon protection layer on the back face of the IGBT until the oxidation is terminated on a gate oxide layer located above the polysilicon protection layer to form a SiO₂ layer, and removing the formed SiO₂ layer and the gate oxide layer by a dry etching process.

The present invention further provides a method for removing a polysilicon protection layer on a back face of an IGBT having a field stop structure, wherein the method comprises the following steps: a) thermally oxidizing a portion of the polysilicon protection layer to form a SiO₂ layer; b) removing the formed SiO₂ layer by a dry etching process; c) repeating the steps a) and b), until the thermal oxidation process in step a) is terminated on a gate oxide layer located above the polysilicon protection layer; and removing the last formed SiO₂ layer and the gate oxide layer by a dry etching process.

The present invention further provides a method for forming an IGBT structure, which comprises removing a polysilicon protection layer and a gate oxide layer by the above mentioned methods, implanting ions into a face where the polysilicon protection layer and the gate oxide layer have been removed so as to form a P+ layer, and depositing a metal formed layer on the formed P+ layer.

According to the method of the present invention, the removal of the polysilicon protection layer avoids the risk of metal contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an IGBT structure having a back face protection layer.

FIG. 2 illustrates an IGBT structure wherein a polysilicon protection layer 12 has been oxidized to form a SiO₂ layer 13.

FIG. 3 illustrates an IGBT structure wherein the SiO₂ layer 13 and a gate oxide layer 11 have been etched off.

FIG. 4 illustrates an IGBT structure including a P+ layer 14 and a metal layer 15.

FIG. 5 illustrates an IGBT structure having a back face protection layer.

FIG. 6 illustrates an IGBT structure wherein a portion of the polysilicon protection layer 12 in the IGBT structure as shown in FIG. 5 has been oxidized to form a SiO₂ layer 13.

FIG. 7 illustrates an IGBT structure wherein the SiO₂ layer 13 as shown in FIG. 6 has been removed.

FIG. 8 illustrates an IGBT structure wherein the last polysilicon protection layer 12 left after multiple times of thermal oxidation in the structure as shown in Fig. has been thermally oxidized to form a last SiO₂ layer 13.

FIG. 9 illustrates an IGBT structure wherein the gate oxide layer 11 and the polysilicon protection layer 12 in the structure as shown in FIG. 5 have been removed.

FIG. 10 illustrates an IGBT structure wherein a P+ layer 14 and a metal layer 15 have been formed in the IGBT structure as shown in FIG. 9.

DETAILED DESCRIPTION

Now a further description of the present invention will be made in combination with the accompanying drawings. Those skilled in the art would appreciate that, the following discussion is merely non-limiting explanation of the subject of the present invention in combination with specific implementations, the scope claimed by the present invention shall be defined by the appended claims and any modification or change without departing from the spirit of the present invention shall fall within the scope defined by the claims of the present invention.

In the descriptions hereinafter, the same layers will be indicated by the same reference numbers in the accompanying drawings.

Example 1

FIG. 1 illustrates an IGBT structure having a back face protection layer. As shown in FIG. 1, the IGBT structure comprises a surface passivation layer 1, a metal layer 2, a dielectric layer 3, a polysilicon layer 4, a gate oxide layer 5, a P+ layer 6, an N+ layer 7, a P-body layer 8, a drift region 9; a field stop layer 10, a gate oxide layer 11; and a polysilicon protection layer 12. According to this embodiment, the polysilicon protection layer 12 is thermally oxidized to become SiO₂ completely, wherein the thermal oxidation is a conventional thermal oxidation method, for example, atmospheric pressure oxidation or high pressure oxidation. Due to a barrier effect of the gate oxide layer 11 (which is an oxide dielectric layer formed by SiO₂), oxidation of the polysilicon protection layer 12 may be terminated because of contact with the gate oxide layer 11 after polysilicon has been oxidized completely. FIG. 2 illustrates an IGBT structure wherein the polysilicon protection layer 12 has been oxidized to form a SiO₂ layer 13. According to this embodiment of the present invention, after the polysilicon protection layer 12 has been oxidized to become the SiO₂ layer 13, the SiO₂ layer 13 thus formed and the gate oxide layer 11 already formed in FIG. 1 are removed by a dry etching process, wherein dry etching may be plasma etching, ion beam burnishing or reactive ion etching (RIE). FIG. 3 illustrates an IGBT structure wherein the SiO₂ layer 13 and the gate oxide layer 11 have been etched off. Afterwards, a P+ layer is formed by ion implantation on the back face, and then, metal deposition is performed on the back face by a method of, for example, evaporation deposition, hence the whole device structure is formed, wherein the ions forming the P+ layer are trivalent ions, such as boron. FIG. 4 illustrates an IGBT structure including a P+ layer 14 and a metal layer 15.

According to the method described above, the polysilicon protection layer is oxidized to form the SiO₂ layer by means of thermal oxidation, which, as compared to the conventional SEZ method, prevents the problem of corroding the field stop layer 10. In addition, as the oxidation of the polysilicon protection layer 12 and the removal of the SiO₂ layer 13 and the gate oxide layer 11 occur before formation of the metal layer, the risk of metal contamination is also avoided.

Example 2

As in FIG. 1, FIG. 5 illustrates an IGBT structure having a back face protection layer. As shown in the figure, the IGBT structure comprises a surface passivation layer 1, a metal layer 2, a dielectric layer 3, a polysilicon layer 4, a gate oxide layer 5, a P+ layer 6, an N+ layer 7, a Pbody layer 8, a drift region 9; a field stop layer 10, a gate oxide layer 11; and a polysilicon protection layer 12. According to this embodiment, a portion of the polysilicon protection layer 12 is thermally oxidized to SiO₂, wherein thermal oxidation is a conventional thermal oxidation method, for example, atmospheric pressure oxidation or high pressure oxidation. FIG. 6 illustrates an IGBT structure wherein a portion of the polysilicon protection layer 12 has been thermally oxidized to form a SiO₂ layer 13. Afterwards, the SiO₂ layer 13 is etched off by a dry etching process, wherein dry etching may be plasma etching, ion beam burnishing or reactive ion etching (RIE). FIG. 7 illustrates an IGBT structure wherein the SiO₂ layer 13 has been removed. As compared to that of the IGBT structure shown in FIG. 5, the polysilicon protection layer 12 of the IGBT structure shown in FIG. 7 has become thinner. A further portion of the polysilicon protection layer 12 in the structure shown in FIG. 7 is oxidized to become the SiO₂ layer 13, and then the SiO₂ layer is removed by means of dry etching. This process is repeated until the last remaining polysilicon protection layer 12 is thermally oxidized finally. Due to a barrier effect of the gate oxide layer 11 (which is an oxidate dielectric layer formed by SiO₂), oxidation of the remaining polysilicon protection layer 12 may be terminated because of contact with the gate oxide layer 11 after polysilicon has been oxidized completely. FIG. 8 illustrates an IGBT structure wherein the last polysilicon protection layer 12 has been thermally oxidized to form a last SiO₂ layer 13. The last SiO₂ layer 13 and the gate oxide layer 11 are removed by means of dry etching to form a IGBT structure as shown in FIG. 9, which, when compared with the structure shown in FIG. 5, does not include the gate oxide layer 11 and the polysilicon protection layer 12. Afterwards, a P+ layer is formed by ion implantation on the back face, and then, metal deposition is performed on the back face by a method of, for example, evaporation deposition, hence the whole device structure is formed, wherein the ions forming the P+ layer are trivalent ions, such as boron. FIG. 10 illustrates an IGBT structure including a P+ layer 14 and a metal layer 15.

Similar to Example 1, as compared to the conventional SEZ method, the method in this example employs a thermal oxidation process to oxidize the polysilicon protection layer to form a SiO₂ layer, thus preventing the problem of corroding the field stop layer 10. In addition, as the oxidation of the polysilicon protection layer 12 and the removal of the SiO₂ layer 13 and the gate oxide layer 11 occur before formation of the metal layer, the risk of metal contamination is also avoided.

The method in Example 2 employs multiple times of oxidation as well as multiple times of dry etching, and therefore, is more applicable to a structure with a thicker polysilicon protection layer.

In a word, the present invention employs a method of thermal oxidation to transform the polysilicon on the back face gradually into SiO₂, and, due to a barrier effect of SiO₂, oxidation of the polysilicon protection layer is terminated on the back face oxide layer. Afterwards, the back face SiO₂ is removed by a dry etching process, which may not only be compatible with the conventional IGBT process and save costs, but also guarantee that the back face FS layer will be not etched, so as to ensure sufficient thickness of the FS layer, hence it is well guaranteed that the performance parameters of device will not be affected. The back face described above refers to a face on which the metal layer 15 is formed. The back face SiO₂ refers to the SiO₂ layer 13 described above. 

What is claimed is:
 1. A method for removing a polysilicon protection layer on a back face of an IGBT having a field stop structure, comprising: thermally oxidizing the polysilicon protection layer on the back face of the IGBT until the oxidation is terminated on a gate oxide layer located above the polysilicon protection layer to form a SiO₂ layer; and removing the formed SiO₂ layer and the gate oxide layer by a dry etching process.
 2. A method for removing a polysilicon protection layer on a back face of an IGBT having a field stop structure, comprising the following steps: a) thermally oxidizing a portion of the polysilicon protection layer to form a SiO₂ layer; b) removing the formed SiO₂ layer by a dry etching process; c) repeating the steps a) and b), until the thermal oxidation process in step a) is terminated on a gate oxide layer located above the polysilicon protection layer; and removing the last formed SiO₂ layer and the gate oxide layer by a dry etching process.
 3. The method of claim 1 in combination with a method for forming an IGBT structure, comprising: implanting ions into a face where the polysilicon protection layer and the gate oxide layer have been removed so as to form a P+ layer; and depositing a metal formed layer on the formed P+ layer.
 4. The method of claim 2 in combination with a method for forming an IGBT structure, comprising: implanting ions into a face where the polysilicon protection layer and the gate oxide layer have been removed so as to form a P+ layer; and depositing a metal formed layer on the formed P+ layer. 